The present invention relates to a process for the preparation of the phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt which is largely free from impurities.
The preparation of the phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt is known per se (DE A 440 96 89). The process can be explained by the following equation: 
Phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt is then carried out at a temperature of about 120xc2x0 C. at a reaction time of one hour. Hydrolysis in ice water is then carried out, phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt crystallizing out; the crystallizate is filtered off. The still moist crystallizate is then taken up and dissolved in an alkaline hydroxide solution, such as sodium hydroxide solution, then treated with activated carbon and heated. The activated carbon is filtered off, and the phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt is precipitated out by adding sulfuric acid. Drying gives pulverulent phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt with a purity of 99% by weight.
During the performance testing of the phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt prepared in this way, it is found that the use of this product prepared in accordance with the above process leads to unacceptable discolorations in the formulations.
A cause of the discolorations are, inter alia, diphenylamine, 3,4-diamino-benzenesulfonic acid and 2-(4xe2x80x2-carboxy-phenyl)-benzimidazole-6-sulfonic acid.
An object of the present invention is the preparation of phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt in high purity which, in particular, does not contain any discoloring components.
We have found a process for the preparation of phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt by reacting o-phenylenediamine with terephthalic acid and chlorosulfonic acid in the presence of strong acids, which is characterized in that the reaction time is 10 to 15 hours.
The process according to the present invention gives phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt in a purity greater than 98% by weight. By-products are merely 
The by-products are safe and cause no discoloration of the phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt.
The preferred reaction time for the process according to the present invention is 11 to 12 hours.
In a preferred embodiment of the process according to the present invention, the phenylene-bis-benzimidazole-tetrasulfonic acid obtained in the reaction or after hydrolysis is, in a first step, dissolved in water and treated with activated carbon, which is then separated off, and where the phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt is precipitated out of the filtrate with sodium chloride and separated off, and, in a second step, is again dissolved in water and sodium hydroxide solution and again treated with activated carbon, which is then again separated off, where pure phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt precipitates out of the filtrate by acidification and is then optionally also purified.
The phenylene-bis-benzimidazole-tetrasulfonic acid obtained in the reaction or after hydrolysis is dissolved in the first step in water in the temperature range from 40 to 80xc2x0 C., preferably 45 to 55xc2x0 C.
Preference is given here to preparing a 1 to 30% by weight, preferably 5 to 7% by weight, solution of phenylene-bis-benzimidazole-tetrasulfonic acid.
Activated carbon for the process according to the present invention (first and second step) can be all commercially available types.
To precipitate out the sodium salt, a 1 to 15, preferably 3 to 10, equimolar amount of sodium chloride is generally added.
The phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt obtained in the first step is dissolved in the second step in water in the temperature range from 30 to 80xc2x0 C., preferably from 45 to 50xc2x0 C.
Preference is given here to preparing a 5 to 25% by weight, preferably 15 to 20% by weight, solution of phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt.
The phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt obtained in the first step is dissolved and, after treatment with and removal of the activated carbon, precipitated out by acidification to about pH 3. The acidification is preferably carried out using hydrochloric acid. Surprisingly, the disodium salt precipitates out here as well.
The phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt obtained in the second step can be washed again with phosphoric acid in a preferred embodiment.
The process according to the present invention can be carried out, for example, as follows:
Sulfuric acid, terephthalic acid and o-phenylenediamine are initially introduced and the mixture is heated to, for example, 110xc2x0 C. under a nitrogen atmosphere. Then, over the course of 4 h, chlorosulfonic acid is metered in at a temperature between 110 and 120xc2x0 C., and then the mixture is further stirred for 12 h. The phenylene-bis-benzimidazole-tetrasulfonic acid obtained after hydrolysis is dissolved at 35-40xc2x0 C. in water, treated with activated carbon and stirred for about 30 min at the same temperature. The activated carbon is filtered off, and the filtrate is admixed with sodium chloride and slowly cooled to room temperature with stirring over the course of about 2 hours for precipitation. The resulting phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt is then filtered off and after-washed with 5 percent strength sodium chloride solution.
The resulting phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt is again introduced into water and, for dissolution, 45 percent strength sodium hydroxide solution is added to a pH of 5, then activated carbon is added again. The mixture is maintained at 55xc2x0 C. and stirred for about 2 hours. The activated carbon is separated off and the filtrate is acidified with pure hydrochloric acid to pH 3 to precipitate out the phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt again. The mixture is stirred for about 2 hours and cooled to room temperature. The phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt is separated off in a manner known per se, e.g. by filtration.
The resulting phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt can then also be washed with a 2% by weight aqueous sodium chloride solution which has been adjusted to pH 3 using small amounts of phosphoric acid; drying is then carried out, for example, at 140xc2x0 C. and 2 mbar.
Use of the process according to the present invention gives a product which is analytically perfect in the trace region, which is best suited for use in cosmetic formulations and does not have the discoloration disadvantages detailed in the introduction. Surprisingly, the presence of problematical trace components is prevented.
Phenylene-bis-benzimidazole-tetrasulfonic acid disodium salt can be used as UV-A filters in cosmetic products.